PROJECT SUMMARY Immune checkpoint blockade is a recent cancer therapeutic strategy that has enabled durable responses in 15- 40% of patients for several cancers by licensing CD8+ tumor infiltrating lymphocytes to kill tumor cells. Despite dramatically altering the clinical course in a subset of patients, these drugs fail to elicit durable response in the majority of cancer patients. The human microbiota is thought to regulate immune tone or responsiveness, and thus is a promising, modifiable target to improve checkpoint blockade response rates. Indeed, response rates for epidermal malignancies to anti-PD-1 have been associated with fecal microbial diversity, and patients? prior use of antibiotics. Moreover, particular bacterial isolates have been identified in mice that can disproportionally contribute to tumor immune responses following anti-CTLA-4 and anti-PD-L1 therapy. While the potential importance of microbiota manipulation for checkpoint blockade response is clear, much work remains to be done to determine how these microbiota-dependent immune responses to cancer are generated and maintained. To begin understanding how the microbiota-immune interface in the gut can contribute to the immune activity and anti-PD-L1 response of non-mucosal malignancy, I have established a humanized gnotobiotic model of anti-PD-L1 treated melanoma. I have demonstrated that a defined microbial community can inhibit B16 melanoma response to anti-PD-L1. This grant aims to dissect the yet unknown mechanisms that mediate this microbiota-dependent inhibition of tumor response to anti-PD-L1 by evaluating the functions and phenotypes of several immune cell populations in the tumor and tumor draining lymph node. Aim 1?Based on my preliminary data we have selected two defined microbial communities that result in contrasting tumor growth rates when colonized into germfree, B16 melanoma-bearing mice. I aim to evaluate the influence of intestinal microbiota on anti-tumor T cell responses following checkpoint blockade therapy by exploring possible differences in tumor-immune architecture, modifications to T cell proliferation and killing of tumor cells, depletion of key T cell subsets and regulators in vivo, and metagenomic profiling before and after anti-PD-L1 treatment. Aim 2?The adaptive immune response is dependent upon priming and activation by myeloid cells, which in turn are modified by microbial sensing machinery. We hypothesize that alterations in myeloid cell functions and phenotypes regulate the adaptive response to anti-PD-L1. To evaluate these cells I will preform transcriptomic and proteomic analysis of macrophages and dendritic cells in the tumor and draining lymph node, assess response causal dependency through myeloid subset depletion, and evaluate the priming ability of dendritic cells from responding and nonresponding gnotobiotic mice. By studying these gnotobiotic animals with different clinical responses to checkpoint blockade, we hope to uncover the mechanisms that contribute to checkpoint blockade response, and possibly identify new immunotherapy targets or biomarkers.